For a long time the field of fuel analysis has sought a rapid, accurate and reliable test method that employs a portable sensor for determining physical and chemical properties of a liquid fuel. Desirably, the method and sensor would be applicable to a wide variety of liquid fuels including those derived from conventional fossil fuels, such as diesel and jet propulsion fuels, and those derived from non-conventional resources, such as bio-based renewables and synthetic fuels from Gas-to-Liquid (GTL) and Fisher-Tropsch (FT) processes. As used herein, the term “physical property” is defined as a property of the fuel which upon determination under analytical test conditions does not change the identity of the fuel. In contrast, the term “chemical property” is defined herein as a property of the fuel which upon determination under analytical test conditions changes the identity of the fuel. As determined by the invention described herein, cetane number, carbon content, and carbon-to-hydrogen (C/H) atomic ratio are construed in this instance to be chemical properties. For purposes of this invention, the physical and chemical properties of a substance, taken individually or as a group, will be referenced hereinafter as “physicochemical properties”.
Generally, diesel engines operate well with a fuel having a cetane number (CN) from 40 to 55. Fuels with higher cetane number have shorter ignition delays, providing more time for the subsequent fuel combustion process to be completed. Commercial diesel fuel is mandated by law to meet strict CN specifications, which vary with state and country in which the diesel fuel is sold. Modern day refineries in the United States must produce diesel fuel with a CN of at least 50. Batches of diesel having a cetane number less than the applicable mandated standard are blended with cetane enhancers to ensure meeting the mandated minimum and targets toward increasingly lower fuel emissions.
Under current refinery process protocols, fuel samples are sent for CN analysis and results are typically available in one or two days. This delay in obtaining the fuel's cetane number does not permit expeditious changing of process conditions and regulation of cetane enhancers for specific fuel batches and can lead to entire fuel batches being reworked due to a late discovery of product variances. As a result, cetane enhancers are added regardless of the need or lack thereof in a particular batch of fuel, due to the unavailability of a measurement device and method providing for cetane number analysis with rapid-turnaround. Often, the cetane number of a batch of diesel, measured after blending with the cetane enhancers, is considerably higher than the minimum required limit. These delays and problems could be avoided if the quality and physicochemical properties of batches of diesel fuel could be ascertained quickly and accurately in real-time. Cost savings would be significant inasmuch as cetane enhancers are very expensive, an order of magnitude more costly than current prices of crude oil per barrel.
Logistic fuels, such as liquid distillate fuels, including jet propulsion fuels JP-8 and JP-5, are used across a full range of internal combustion (IC) engines including conventional compression-ignited IC engines, Remote Piloted Aircraft (RPA) applications, and heavy duty compression ignition engines. Unlike commercial diesel, logistical jet fuels have no cetane specification and vary widely in this key measure of combustion property and quality assurance. Cetane number is correlated with a direct measure of ignition delay, which directly affects engine performance including fuel consumption, engine operability, and maintenance requirements. Variations in engine performance could be avoided if the logistic fuels could be evaluated with a portable fast-response cetane sensor.
Likewise, non-conventional liquid fuels have no cetane specification and vary widely in this key measure of combustion property and quality assurance. By “non-conventional liquid fuels”, we include liquid fuels derived from renewable resources, for example bio-based fuels and synthetic liquid fuels derived from F-T and GTL processes. It should be appreciated that non-conventional fuels have different distributions of paraffinic and aromatic constituents as compared to petroleum-based fossil fuels, making it difficult to assess cetane numbers accurately. Yet, due to a growth in bio-based feedstocks and synthetic F-T and GTL processes, more non-conventional fuels are expected to be available to refineries in the future at relatively low cost; and therefore, having such fuels standardized by cetane number or other physicochemical property would be highly desirable.
Testing methods and apparatuses are known for measuring the ignition delay of diesel fuel and correlating the ignition delay with cetane number. A gold standard for such testing is defined by American Society of Testing Methods Test No. D-613 (ASTM D-613) using a single-cylinder four-stroke cycle, variable compression ratio Cooperative Fuel Research (CFR) diesel engine made by Waukesha. The apparatus is large, heavy, and non-portable, and requires high initial investment and high maintenance costs, special operator training, high fuel volume, and considerable testing time. Somewhat less complex testing methods are known, which measure ignition delay in a constant-volume bomb and correlate the ignition delay to a Derived Cetane Number (DCN), as disclosed for example in U.S. Pat. No. 4,549,815. This latter method is found commercially in a Herzog Cetane ID 510 apparatus available from PAC LP and an IQT™ Diesel Fuel Ignition Quality Tester available from Advanced Energy Technologies. These commercial apparatuses operate at high pressure with purified oxygen and require up to 1 liter of fuel for run times of at least twenty minutes, not including apparatus heat-up time. Moreover, the apparatuses measure ignition delay in milliseconds, thereby requiring a high resolution pressure detector.
The art would benefit from discovery of a compact and portable sensor that provides an analytical testing method for measuring the physicochemical properties of a liquid fuel, such as cetane number, carbon content, and C/H atomic ratio. More desirably, the sensor and testing method would provide rapid results in real-time with accuracy and precision (repeatability). It would be beneficial if the method were to consume less fuel than present day analytical methods and were to operate at ambient pressure on air, rather than pressurized oxygen. Such a method would be even more useful if it were capable of standardizing a wide variety of liquid fuels including gasoline, diesel, distillate, and jet propulsion fuels (JP-5, JP-8, Jet A) as well as bio-based renewable fuels and synthetic fuels including F-T and GTL fuels. For refinery applications, a real-time measurement capability would reduce delays and improve commercial fuel yields by performing in-situ real-time input material validation, quality monitoring, and product verification. Other end-user applications for such a sensor would include fuel quality verification, stored fuel integrity, laboratory fuel analysis, and quality checks on spot-purchased fuel. Moreover, a reliable testing method and sensor for determining physicochemical properties of a variety of liquid fuels would enhance modifications in engine parameters and improvements in engine performance across the range of commercial internal combustion engines.